1. Field of the Invention
The present invention relates to power converters, and more particularly, relates to the soft switching power converters.
2. Description of the Related Art
Flyback power converters have been widely used to provide power supplies for electronic products, such as home appliances, computers, battery charger etc. For achieving higher efficiency and reducing power loss, the power converter can be designed to operate at the quasi-resonant (QR) switching when the power converter is operated at high input voltage and high switching frequency. The quasi-resonant switching is preferred for reducing the switching losses and EMI. However, the drawback of the quasi-resonant power converter is its variable switching frequency. The quasi-resonant switching frequency is changed in response to the change of the input voltage and the output load. In many applications, the specific frequency bands are not acceptable due to the interference issue. The burst mode switching is an approach to avoid the specific switching frequencies.
The object of the present invention is to provide a control circuit that can adaptively operate the power converter at the quasi-resonant switching and the burst mode to achieve higher efficiency and prevent the system being interfered. The detail skills of the flyback power converter can be found in textbook, such as Keith H. Billings, “Switchmode Power Supply Handbook”, McGraw-Hill Book Co. December 1989; and Abraham I. Pressman, “Switching Power Supply Design”, McGraw-Hill Book Co., December 1991. The description of the QR power converter can be found in the prior art of “Switching control circuit having a valley voltage detector to achieve soft switching for a resonant power converter”, U.S. Pat. No. 7,426,120.